6. Ulmus rubra Muhl. (slippery elm)

U. fulva Michx.

Pl. 570 c–e; Map 2668

Plants trees to 35 m tall. Twigs tan to reddish brown or purplish brown, hairy, never with corky outgrowths or wings. Winter buds 4–8 mm long, ovoid to narrowly ovoid, bluntly pointed, dark red to orangish brown, at least the inner scales densely pubescent with short, spreading and shaggy, red to brownish red hairs (these sometimes becoming bleached in late winter). Petioles 4–9 mm long. Leaf blades 10–19 cm long, 5–10 cm wide, broadly oblanceolate to obovate or elliptic, the base strongly asymmetric, tapered to short-tapered to a sharply pointed tip, the major marginal teeth 1.5–3.0 mm deep, blunt to more or less sharp, most with 1–5 smaller secondary teeth, the upper surface strongly roughened, the undersurface moderately to densely hairy along and between the main veins, also noticeably tufted in the vein axils, the secondary veins 10–20 on each side of the midvein, those of many leaves with 3 or more secondary veins on each side of the midvein forked toward their tips; juvenile leaves unlobed or with 1 or 2 tapered lobes per side toward the tip. Inflorescences dense clusters appearing before the leaves develop in the spring on second-year twigs. Flowers sessile or nearly so, the calyces shallowly 5–9-lobed, the tube pubescent with straight white hairs, the lobes broadly rounded, densely pubescent with red to reddish brown hairs on the outer surface and margins. Fruits 1.4–1.9 cm long, 1.2–1.8 cm wide, nearly circular to broadly elliptic, pale tan (usually darker over the seed), the body densely short-hairy, the wings glabrous or minutely hairy on the surfaces, but the margins glabrous. 2n=28. March–April.

Common nearly throughout the state (eastern U.S. west to North Dakota and Texas; Canada). Bottomland forests, mesic upland forests, bases, ledges, and tops of bluffs, banks of streams and rivers, and margins of ponds and sinkhole ponds; also old fields, fencerows, strip mines, roadsides, and disturbed areas.

This is the most abundant species of Ulmus in the state. The inner bark is very mucilaginous, unlike that of our other elms. Native Americans had multiple uses for U. rubra, ranging from a variety of medicinal applications to a kind of tea, a food preservative, cordage, and building material (Moerman, 1998). Slippery elm inner bark is still used medicinally for soothing sore throats, treating colds, and sometimes for lower digestive problems like diverticulitis. It is among the natural products that is wild-harvested, particularly in the Ozarks and sold by individuals to wholesale distributors.

Ulmus rubra hybridizes with U. pumila where the two species grow together, and the hybrids are fertile and backcross to the parental species (Zalapa et al., 2009). Such hybrids occur in St. Louis County, but specimens observed thus far are all vegetative and display juvenile leaf morphology. Some specimens in herbaria have been annotated with the name “U. ×notha,” but this binomial has never been validly published.